Tuning Basics

The cavity set pictured below.
I've made up the loops, set them up for 146.9MHz pass and 146.3 notch, make up all the 1/4 and 1/2 wave patch leads, connect it all up, this chain works, a treat. But, the other set, individually look good but together, the pass is a double hump and 6db down. What do I do?? I tried putting a bandpass cavity between the two BP-BR cavitiies, no effect. I tried a 5db pad from the VNA, no effect.

OK then, how do we fix it? PocketVNA 10db loss double hump, bad

Here we must remember, when we check a single cavity with our two port analyser, we terminate the output connector of the cavity with the input of the analyser. In my case a VSWR of 1.5:1 at 147MHz.
When we connect two cavities in series, we are terminating the first cavity with perhaps a 20:1 VSWR at the notch frequency.
How this effects various cavity coupling loop types is, largely why I'm on this project.
At the end of the day, we want to know what works and is easy.
And, to know WHAT DOESN'T WORK or is HARD. This is MOST important.
The secrets are here actual Vari Notch tuning by Jeff WN3A.

2018, my ex. 70cm 3/4 wave cavity set, ready for 2m conversion.

PocketVNA measurements

Magnetic coupling, Type 3, RHS picture above.

Magnetic coupling, Type 3, Lower picture above.
Both can be done, pass then notch, notch then pass.

Magnetic coupling, Type 3, Lower picture above.
This is with about 4" of connectors added inline from the calibration point.

More detail on www.unixservice.com.au/hamradio/repeaters/duplexers/2Mnotes.html

References:

a) A 1972 article on 2" heliax as resonators for 6m
b) A 1965 IEEE article by Phelps Dodge
c) A general article by EMR Corp
d) A 2010 article by Sinclair
e) Where the loop cannot be rotated...VE2AZX
f) Motorola T1500 UHF series, with internal pictures.
g) TX-RX Coupling Loop Project on the Repeater Builder website but no followup on a full duplexer build.
h) TX-RX-instruction-manual-vari-notch-duplexers-with-6-inch-cavities.pdf 1/4 wave inter-connect cables, not quite correct.
i) andrew-about-rf-communications.pdf Section 4, a general article.
j) Hybrid rings from 1966
k) Vari-Notch patent
l) Chapter on the Hybrid Ring by W2EUP
m) Duplexers, Theory, and More by I1WQR
n) My page on the Hybrid Ring

Alan VK2ZIW